Crystalline polypropylenes produced by using a stereoregular olefin polymerization catalyst are excellent in rigidity and heat-resistance, but have a poor impact resistance, particularly that at low temperatures, which leads to limited use in various fields. As a method to provide improved impact resistance at low temperatures, there has been proposed a process to block copolymerize propylene with other .alpha.-olefins such as ethylene. However, the resulting block copolymers are more improved in impact resistance at low temperatures than crystalline polypropylenes, but provide lowering in rigidity, hardness, heat resistance, transparency, whitening resistance, gloss, tensile elongation and the like, whereby the use thereof is restricted.
In order to solve the above-mentioned problems encountered in the block copolymers, a number of methods have been proposed, for example, a method which comprises a first step of homopolymerizing propylene in the presence of a catalyst for a streoregular polymerization, a second step of copolymerizing a mixed monomer of ethylene and propylene, and then stepwisely repeating the propylene homopolymerization and the ethylene-propylene copolymerization. In relation to the above-mentioned multi-stage polymerization method, Japanese Patent Kokai 54-152095 discloses using a titanium trichloride solid catalyst, and Japanese Patent Kokai 58-201816 discloses using an organoaluminum compound and an electron donating compound in combination with a titanium tetrachloride solid catalyst.
There have also been proposed a propylene block copolymer consisting of a crystalline polypropylene block and an ethylene-propylene random copolymer block, wherein the crystalline polypropylene block content is 55 to 95 percent by weight and the intrinsic viscosity ratio of the both blocks as well as the glass transition temperature of the ethylene-propylene random copolymer block are restricted; and a polypropylene block copolymer which consists of a polymer block mainly comprising propylene and an ethylene-propylene random copolymer block, wherein the intrinsic viscosity ratio of the both blocks and the intrinsic viscosity of the ethylene-propylene random copolymer block are restricted, and wherein the resulting block copolymer is melt kneaded.